This invention relates to a carcinostatic agent having an immunopotentiating carcinostatic effect which contains as an active ingredient Eisenin which is a tripeptide obtained by extraction from seaweed Eisenia bicyclis or by synthesis, and more specifically, it relates to a carcinostatic agent having an immunopotentiating carcinostatic effect which contains as an active ingredient Eisenin (tripeptide) of the general formula: EQU L-Pyroglu-L-Gln-L-Ala
wherein Pyroglu, Gln and Ala represent pyroglutamic acid, glutamine and alanine, respectively.